With increasing development in use of communication technology, multicast service (e.g. IP TV network television, IP conference television service, IP online courses and etc.) has increasingly become a widespread and public service, which is becoming noticeable business point of growth.
FIG. 1 is a schematic diagram of a typical IP multicast architecture. A multicast routing protocol is used between a local router and a remote router, and can be Distance Vector Multicast Routing Protocol (DVMRP), Protocol Independent Multicast-Dense Mode (PIM-DM) and etc. This protocol form a routing forwarding table for multicast based on topology for interfaces entering a multicast group. A group membership protocol is used between an IGMP host and the local router. For example, a typical Internet Group Management Protocol (IGMP) host notifies the local router by this protocol that it desires to enter a certain multicast group and receive messages thereof, while the local router periodically queries whether a member of a known group within a local area network is in an active state (i.e. whether there are still some members belonging to a certain multicast group in this network section) to create and maintain membership information in a direct interconnection section of the router. Also, in order to effectively suppress diffusion of multicast data on a link layer, multicast protocols such as IGMP Snooping/Proxy are introduced into a network device of an access layer.
As shown in FIG. 2, a network access device DSLAM (Digital Subscriber Line Access Multiplexer) effects the functions of IGMP Proxy/enhanced IGMP Snooping. IGMP Proxy/enhanced IGMP Snooping receives a multicast membership report message from a user interface, analyzes IP addresses of multicast groups in the message, create a multicast forwarding table for the IP address of a respective multicast group and the user interface, and forwards the message to an upper layer device. Multicast service streams are replicated to a plurality of network interface based on the multicast forwarding table, and each network interface can be connected to several user terminals (e.g. STB, PC terminals) via a Customer Premises Equipment (CPE). Multicast service often has relative higher requirement for response speed of devices due to its high real-time requirement and high service quality requirement for being directly oriented to terminal users. For example, the device is required to be ms level with respect to processing on request for terminal multicast users leaving. There exist two versions of IGMA for use (IGMAv1, IGMPv2). For a user of IGMPv1, the terminal can enter the multicast group without Leave message. For a user of IGMPv2, it is comprised of Leave message and allows quickly reporting termination of members to the network device. For a user of IGMPv1, when channels are switched for the user, the network access device receives request for entering other channel from the user, but it can not determine whether the user has left the previous multicast service channel or there are other users receiving data from the previous multicast service channel at present. Now the approaches which can be used are mainly two types: A) rule of latest-request-first: DSLAM deletes the interface from the multicast group of the multicast service table and in turn the multicast data stream stops. The shortcomings of this approach are apparent in spite of its high leaving speed: first, the system sets a latest-transmitted message for entering the multicast group from the user as high priority so as to affect the multicast service which the user originally makes request to enter; second, when there are a plurality of users of the multicast on the same interface, one user newly entering the channel will have an effect on other users using the channel. B) General Membership Query (GMQ) process including querying whether there are users on the interface, still transmitting the multicast data stream to the user interface if there are other users responding to this querying; otherwise DSLAM deletes the user interface from the multicast group of the multicast service table and in turn the multicast data stream stops. The GMQ process usually requires a period of response time according to specification of the protocol, while it is directed to all multicast groups, and thus it is possible to result in a great amount of interactions of IGMP protocol packets, which increase burden of the network and affect performance of the system. For a user of IGMPv2, when channels are switched for the user, an IGMA message, which usually has one Leave message followed by two or three Join messages, is transmitted to the DSLAM interface connected to the user. After DSLAM receives the Leave message, two processes as follows can be used in accordance with RFC2236: A) quick-leave approach: DSLAM deletes the interface from the multicast group of the multicast service table and in turn the multicast data stream stops; in this approach, despite high leaving speed, when there are a plurality of users of the multicast on the same interface, leaving of one user will have an effect on other users on use. B) process of entering Group Specification Query (GPQ) including querying whether there are still other users of the multicast group on the interface, still transmitting the multicast data stream to the user interface if there are other users responding to this querying; otherwise DSLAM deletes the user interface from the multicast group of the multicast service table and in turn the multicast data stream stops. The GSQ process usually requires a period of 1-2 seconds according to specification of the protocol, so that when channels are switched for the user, the response speed of the system is relatively low, the previous multicast data stream can not stop to release interface resource and the newly-added multicast data stream can not be provided to the user immediately if the interface resource is limited at that time (e.g. due to bandwidth limit), which will have an effect on service experience of the user.
An improved method for channel switching in IGMP service provided in U.S. Pat. No. 7,228,356, entitled “IGMA Expected Leave Triggered BY MAC Address”, which determines whether the Leave message is transmitted by means of tracing and comparing MAC information of users. However, this solution has the following defects: 1) the user information is traced and recorded only when the user enters, and when there are a plurality of users, the items are too many, which occupy precious memory space in operation; 2) the tracing and recording table of the user is so long that it takes too long time to search for match, so delay performance will be affected when the user is switched to leave; 3) this solution can not effectively solve the problem of quick-leaving for IGMPv1 since there is no Leaving message when the user of IGMPv1 leaves multicast; 4) when switching-back of channels is performed on users, the previous leaving multicast data stream need to be re-established.
Thus it can be seen that an effective means for multicast service of user interface management is lacked in the existing network access device.